1. Field of the Invention
The present invention relates to a wiring board and a magnetic disk apparatus, and particularly relates to a wiring board that is preferable for a small magnetic disk apparatus and the like and a magnetic disk apparatus having such a wiring board.
2. Description of the Related Art
Along with the sophistication in performance and the reduction in size and weight of portable devices and electronic devices, miniaturization is required also in a magnetic disk apparatus as a representative apparatus for storing a large volume of digital information. For such a small magnetic disk apparatus, the size of a disk enclosure itself storing a disk and so on is small, and each wiring board equipped therein is also required to respond to miniaturization.
In a wiring board as described above, in order to respond to narrowing down of a pitch between electrodes, solder balls are often used for connecting electrodes of an electronic part and electrodes on the wiring board when mounting an electronic part such as a semiconductor chip on the wiring board. In this case, if a force such as stress during temperature cycling caused by difference in coefficient of linear expansion between the wiring board and an electric part, dropping impact, bending or the like is applied, concentration of stress occurs at solder junction points, which may cause connection failure. Accordingly, it is known to fill an underfill constituted of resin or the like into a space between the electronic part such as a semiconductor chip and the wiring board so as to reinforce the solder junction points. Further, in order to stop the flow of this underfill, it is known to form a frame constituted of copper foil that the same material as the electrode (for example, refer to Japanese Patent Laid-open Application No. 2002-110712).
In order to respond to miniaturization of magnetic disk apparatuses, further microfabrication of wiring patterns in a wiring board is in progress. Conventionally, such a wiring board is fabricated by a method in which a cover layer (polyimide layer for example), conductor layer, reinforcing plate and the like are formed respectively in a predetermined shape by metal molding or the like and bonded together by an adhesive. However, in such a fabricating method, owing to displacement of bonding, limitation in design due to minimum processing restriction by a metal mold and the like, demand for high density design that is required in a wiring board for miniaturized magnetic disk apparatus cannot be satisfied.
Thus, the present inventors have developed a new fabricating method using a new material, that is, a fabricating method in which a liquid photosensitive polyimide is used to repeatedly perform exposing, developing, curing and so on in sequence so as to form respective layers constituting a wiring board sequentially. According to this fabricating method, the metal molding is not necessary and the displacement of bonding does not occur, so that the demand for high density design can be satisfied.
However, when the material different from that of a prior wiring board is used to fabricate a wiring board as described above, it has been found that a problem arises such that difference in wettability of the surface of a wiring board is generated, and thus the underfill diffuses circumferentially when it is filled into the space between the electronic part and the wiring board, which results in a high possibility of causing production failure.